Variable gain amplifier



June 24, 1941. D. a. SMITH VARIABLE GAIN AMPLIFIER Filed may e, 1937 I II Patented June 24, 1h41 Eiwit VAREABLE GAIN AMPLIFIER David B. Smith,Philadelphia, Pa., assigner to Philco Corporation, a corpcration ofPennsylvania Application May 6, 1937, Serial No. 141,151

13 Claims.

This invention relates to the control of the gain or amplication ofvariable gain ampliners and, more particularly, to a novel gain controldevice by means of which an input signal may be amplified to an extentwhich is governed by a second electrical signal. The invention is'capable of general application and may be used in any instance Where itis desired to control the amplification of a signal in the manner abovementiond. For example, the invention is Well adapted for use intelevision systems, and finds several different applications therein. Inorder to illustrate the invention, a specific example of its use for oneparticular purpose in a television system will be given. It will beunderstood, of course, that speciiic embodiments of the invention otherthan those disclosed herein may be used and are, therefore, to beconsidered within the scope of the invention.

One object of the invention is to provide an amplifier having a highimpedance input circuit, which when energized by one or more inputsignals, will form in its output circuit, an output signal proportionalto the input signal or to the difference between the input signals.

Another object of the invention is to provide a variable gain amplieradapted to transfer signals over a Wide frequency range, and comprisinga plurality of groups of space discharge devices, the devices of onegroup being adapted to supply input signals of opposite phase to thedevices of another group which are so arranged as to obtain an outputsignal that is proportional to the difference between the inputsi-gnais. Consequently, a control signal applied in the saine phase tothe devices of said one group balances out in the system and does notappear in the output circuit but serves only to control the gain of thedevices of said one group.

A further object of the invention is to provide a system of thischaracter wherein the space discharge devices of said one group comprisea cathode load feature and wherein there is provision for loading theanode circuits of said devices to the full extent to obtain fulladvantage of the cathode load feature.

Still another object of the invention is to provide an ampliiier havingone input circuit which may be adapted for push-pull input signais, andanother input circuit for a control signal, the amplifier being soarranged that, first, the control signal is definitely established insuch a manner as to control the mutual conductance and thus the gain ofthe amplifier tubes without introducing degeneration of the inputsignals and, second, the output signal of the amplifier comprises onlythe input signal amplified by an amount dependent upon the controlsignal.

Other objects and features of the invention will appear hereinafter asthe description proceeds.

In the accompanying drawings:

Fig. 1 is a circuit diagram of a device embodying the invention;

Fig. 2 shows the characteristic curves of certain tubes which may beused in the device of the invention; and

Fig. 3 is a schematic illustration of a scan ning signal generatoremploying an embodiment of the invention which may be used to actuatethe scanning means in a conventional television system.

The generaI principles of the invention may be understood from aconsideration of Fig. l wherein the output signal En appearing in theoutput circuit corresponds to the amplified input signal E1, the degreeof amplification depending upon. a second or auxiliary input signal E2.The device comprises two tubes V1 and V2, which may be supplied withinput signals by the phase-inversion tube V3. Such phase-inversioncircuits are well known in the art and need not be described in detail.It suices to state that signals of opposite phase are applied to tubesV1 and V2. These tubes have their cathodes connected in a common cathodecircuit including the resistor R1. Tubes V1 and V2 are preferablyidentical, one with the other, with respect to their electricalcharacteristics. The average bias Voltage applied to the grids Aof thesetubes are maintained at corresponding values with respect to somepredetermined voltage level, such as ground, by means of the biasbattery C and the resistor R. Due to this resistor, the opposite phasevoltages applied to the Igrids of tubes V1 and V2 nuctuate about thepredetermined bias lvoltage level in response to the signal E1. Thecathodes of tubes V1 and V2 are maintained at some voltage greater thanground potential, depending upon the voltage drop across the resistanceR1. Thus the input signals for tubes V1 and V2 comprise the bias voltagedue to battery C and the voltage drop in the resistor R1 plus thealternating components of the signel E1, which cause the grid voltagesof tubes V1 and V2 to iiuctuate about the bias voltage level.

As described more particularly hereinafter, the two signals of oppositephase applied to the input circuits of tubes V1 and V2 produce an outputsignal E0, but any signal voltage appearlthe same as the impressedvoltage.

ing across the resistor R1, which will be applied in the same phase totubes V1 and V2, will not produce an output signal. Consequently, thesignal E1 will be transferred through the system, but variations in thebias voltage of the tubes V1 and V2, which will appear as input signalsof the same phase to both tubes, will not be transferred through thesystem. Tubes V1 and V2 may have the characteristic that their mutualconductance, which is a measure of gain or amplification, is a functionof the input voltage. The amplification of the system may then be variedby varying the voltage drop across the resistance R1 which will vary themutual conductance of tubes V1 and V2 and, consequently, will vary thedegree of amplification of the signal E1. The voltage drop across theresistance R1 may be conveniently controlled by means of an additionaltube V4 whose cathode is connected in common with the cathodes of tubesV1 and V2 and whose anode is connected to the plate supply source B.Thus, the current drain of tube V4, which may be used to control thevoltage drop across the resistance R1, may be varied readily by varyingthe input signal E2 applied to the tube. It will be seen, therefore,that the input signal E1 may be amplified in the device by an amountdependent upon a second or auxiliary input signal E2 to obtain an outputsignal En similar in wave shape to the input signal E1 and containing nocomponents directly proportional to the auxiliary signal E2.

As will be apparent from considering Fig. 1, the resistance R1 isincluded in both the input and output circuits of tubes V1, V2 and V4,and the voltage thereacross is included in the total voltage in theinput circuits of these tubes as well as in the output circuits of tubesV1 and V2, and in addition this voltage may constitute the total outputvoltage of tube V4.

In the preferred mode of operation of the device, the signal E1 will besmall as compared with the signal E2. Further, in general, the combinedspace current of tubes V1 and V2 Will be of the same order as that oftube V4. The cathode drive of tube V4 which biases tubes V1 and V2 maythen offer certain unusual advantages. It will be seen that when asignal is applied to the input circuit of tube V4, the tube will tend todraw enough current so that the voltage across R1 is Under theseconditions, the actual grid-to-cathode or tube input voltage would besimply the biasl voltage. The tube will not, in general, however, drawsufficient current to bring about this condition, but will fail by thediierential grid voltage required to cause the change in space current.In other words, the tube is not actuated by the input voltage but by thedifference between that voltage and the voltage obtained across theresistor R1. By designing the circuit so that a small change in actualgrid voltage of tube V4 will cause a large change in the voltage dropacross the resistor R1 due to the change in space current of tube V4,the actual voltage across R1 may be made to follow the input Voltage E2very closely.

It will be observed that the operating range of input voltages may beconsiderably larger than the total rated grid swing of the tube V4,since the actual grid-to-cathode voltage is the difference between thevoltage E2 and the Voltage drop across the resistor R1, neglecting, ofcourse, the unidirectional component which may be controlled by theaverage bias on the grid of tube V4.

Thus, for example, the actual grid-to-cathode Voltage on tube V4 may bea small fraction, say 1/10, of the actual input voltage. Since thecircuit may be designed so that the tube operates over only part of itsgrid voltage-plate current characteristic, signal distortion due tonon-uniformity of tube characteristic may be minimized to a considerableextent.

It is worthy of note that degeneration of the signal E1, which isusually present when an appreciable impedance is introduced in thecathode circuit of a vacuum tube amplifier having an anode load circuit,is avoided due to the property of this circuit of balancing out signalsintroduced in both branches. It should be noted further that the eiectof variations in space current of tubes V1 and V2, which variations aresmall, is further materially reduced by the use of the cathode load fortube V4. Since this tube is actuated by the difference in voltagebetween the input signal E2 and the voltage drop across the resistor R1,which difference voltage Will be small, it makes little differenceWhether the latter voltage is due to space current in tube V4 or to someother cause, and in any event, the tube V4 will operate to minimize thediscrepancy in voltage between signal E2 and the resistor voltage. Thus,the circuit of tube V4 not only establishes the bias on tubes V1 and V2in accordance with the signal E2, but it also functions to minimize anytendency of these tubes to have any otherbias, and thus minimizes anydegeneration of the signal E1 due to the cathode resistor R1.

In order to obtain full advantage of the cathode load, it is desirablethat tubes V1 and V2 have an appreciable load in their respective anodecircuits. The invention provides for loading of the anode circuits tothe full extent. Tubes V1 and V2 are supplied with load impedances R3and R4, respectively, which are of suitable magnitudes to act as properloads for their respective tubes. The amplied signals obtained across R3and R4 are supplied through blocking condensers to the grids of tubes V5and V5. The cathode of tube V5 may be connected to the plate of tube V5,the anode of tube V5 may be connected to the B supply, and the cathodeof tube V5 may be connected to ground. If desired, the plate voltages oftubes V5 and V5 may be increased by connecting the cathodes of tube V5to the negative side of a second B Voltage supply unit whose positiveside is connected to ground. The common junction of the tubes V5 and V5is connected to an output load resistance R5 which may be connected toan intermediate point on the whole B voltage supply. Obviously, thisintermediate point might alternatively be provided by a drop wire orother similar device. Tubes V5 and Vs should preferably have identicalelectrical characteristics, and their respective input grids may besuitably biased to similar operating points by the grid-leak resistorsR5 and Rv, as shown.

Tubes V5 and V5 are thus connected so that their output circuits are inseries with each other and the battery B. 'Ihe output circuit for thetubes, however, is the resistance R5 which for A. C. components iseiiectively connected across each tube output circuit. Signals across R4are supplied to the input circuit of tube V5 while signals across R3 arecombined With the output voltage across R5 and then supplied to theinput circuit of tube V5.

By this interconnection of input and output circuits, a circuit isobtained such that the output signal is proportional to the differencebetween the input signals, and yet the system is perfectly stable.

This result is due to the fact that as far as results are concerned theoutput circuit has the properties of a cathode load output circuit, butthe circuit interconnections are such that signals introduced in oneinput circuit are transferred without any phase reversal, while signalsin the other circuit are transferred reversed in phase. Thus signalssupplied to the input circuit of tube Vs are reversed in phase andtransferred to the output circuit from which they are fed back to theinput of tube V5 which tube operates to control the amplitude of theoutput signal so that it will be the same as that due to an input signalsupplied to tube V5 which signal, however, would be transferred to theoutput circuit without reversal of its phase. Thus the output signalwill be proportional to the dilierence between the input signals. Forexample, in the case in which the same signal is applied in each inputcircuit, the current in each tube would change by the same amount andthere would b-e no current change in R5 since the current through thisiinpedance is the difference between the current in each tube, Whereas,if equal and opposite signals were applied to each input circuit, thecurrent in one tube would increase while that of the other woulddecrease, the dilference current through R5 would produce an outputsignal which would feed back affecting the grid voltage of the one tubeand the plate voltage of both in such a way as to cause equalamplification of both input signals. The resultant output signal wouldthen be proportional to twice the input signal.

Consider now the operation of the circuit as a whole. The, group oftubes V1 and V2 may be biased to the same point by means of the commonresistance R and the common Voltage across R1. age across R1 andsupplied in opposite phase to tubes V1 and V2 by means of the phaseinverter V3 as shown. Output signals from tubes V1 and V2 may be derivedfrom resistances R1 and R4 respectively and supplied to the group oftubes V5 independent of tub-es V1 and V2 and determined only by thecontrol signal E2. The grid-to-cathode voltage of tube Vs willcorrespond to the voltage across Ri while that of tube V5 will be thevoltage across R3 combined with that across R5.

Preferably R3 and R4 should have the same value. and V5 and Ve should besimilar tubes. Under these conditions, the output signal across R5 willbe proportional to the difference signals supplied to the group of tubesV1 and V2, and the proportionality factor will be dependent upon thecontrol signal E2 which will not otherwise appear in the output circuit.The input circuits of tubes V5 and Vs constitute high impedance sourcesand consequently do not tend to restrict the use of proper loadimpedances R3 and R1 in the output circuits of tubes V1 and V2. Hence,the optimum operating conditions for the circuit as a whole may beobtained.

Thus, an input signal is supplied to one of two amplifying branches eachof which includes an amplifier tube whose mutual conductance is afunction of its grid voltage. A similar signal of opposite phase issupplied to the input circuit of the other branch. A second input signalis supplied to the input circuits of both branches in A signal E1 may becombined with the voltif such a way that the mutual conductance of anamplifier tube in each branch is modified, producing in each branch anidentical undesired signal and determining the degree of amplificationof the signal E1. The output of each of these branches is then combinedin a circuit which forms in its output circuit a signal proportional tothe difference between the two input signals, that is, a signalcorresponding to E1 but amplified by an amount determined by E2, andwhich permits the use of load impedances in the anode circuits of theinput tubes, thus enabling better control of the gain of these tubes bythe control tube.

While it is preferred to apply opposite phase input signals to tubes V1and V2, as in Fig. 1, it will be apparent that an input signal appliedto one of these tubes only will be transferred through the system whilea common control signal will be balanced out. It is contemplated,therefore, to apply an input signal to only one of the tubes V1 and V2,if desired.

The amplifier shown in Fig. 1, and described above, is adaptable for useover a wide frequency range due to the fact that, in general,resistancecapacitance coupling is used and, as well known, such couplingdevices are substantially uniform over a frequency range extending fromsome low Value to reasonably high values. It will be noted that thereare no transformers employed in the device of Fig. 1 which would tend torestrict the frequency range. The use of the phase inverter Vi makes itpossible to obtain the opposite phase input signals for tubes V1 and V2without employing a transformer.

It is likewise important that the frequency response of each branch bethe same at the low frequency as well as the high frequency end. andhence in Fig. l the values of the condensers and resistors connected toR3 and R4 should be so designed that the transfer circuits will havesimilar frequency characteristics. As R3 and Ri should preferably beequal, this may easily be accomplished by using condensers and gridleal: resistors of equal values.

It is worthy of note that in the circuit of Fig. l., no unusualrestrictions are placed upon the source of B voltage for the circuits,and the same B supply as that used for associated apparatus may beemployed. In addition, one side of each of the several input and outputcircuits may be grounded for A. C. components which facilitates the useof the circuit.

The variable gain amplifier provided by the invention finds several usesin television systems where wide frequency ranges are encountered. Forexample, due to diiference between the center line of the optical systemof a television carnera tube and the axis of the electron beam generatorwhich supplies the scanning beam, an effect known as the keystone effectoccurs and this effect may be overcome by causing the amplitude of thehorizontal deflection of the beam to vary in accordance with the amountof vertical deiiection. This may be accomplished by amplifying thesignal for producing horizontal deflection in a variable gainanuoliiier` the gain of which is determined by a second signal derivedfrom the vertical deiiecting system. In this instance it is desirable tohave the amplitude of the signal E1 substantially directly proportionalto a constant plus the signal E2. This may be accomplished by using forV1 and V2, tubes having the mutual conductance characteristic MZ shownin Fig. 2. If, for example, the signal E2 varies the operating points ofthe tubes V1 and V2 from the -grid voltage value GV1 in Fig. 2 to thevalue GVz, the amplincation or gain of the amplified signal E1 will varyfrom pmi to gmc since the stage gain is substantially proportional tothe mutual conductance of the tube.

In another instance it may be desirable to have the gain constant at onevalue part of the time, and zero or some low value other times. This maybe done by using for V1 and V2, tubes having a mutual conductancecharacteristic such as N in Fig. 2. In this case the control. signal E2may vary the operating point from GV4 to GVg for which points the gainwill be zero and proportional to gms, respectively. It will be notedthat if the gain curve N is flat topped from to y, the voltage of thecontrol signal plus the input signal Et may be anywhere in the intervala.' to y, thus permitting the use of larger input signals Ell. Where apush-pull input circuit such as that of Fig. 1 is used, distortion ofthe signal El due to the slope of the gm curve is largely balanced out,and larger input signals maybe employed. Tubes having characteristics asshown in Fig. 2 are now available commercially. For example, tubes ofthe type known as 42 have a characteristic similar to M while tubes ofthe type known as '77 have a characteristic similar to N. Other tubeshaving similar or different characteristics may, of course, be used,depending upon the particular application.

In Fig. 3, there is shown in block diagram a schematic outline of acircuit which may be used to avoid the keystone effect above mentioned.In the figure, horizontal and vertical deflecting signals En and Evrespectively are supplied to blocking tube oscillators l and 2 which inturn energize generators 3 and i which form the proper scanning signals.In the vertical signal circuit, the blocking tube oscillator t alsoenergizes a sav/tooth signal generator 5. The output oi the generator 5is supplied to a variable gain amplilier 6 of the type shown in Fig. 1which amplifier is included in the horizontal signal circuit and servesto control the amplitude of the horizontal scanning signal in accordancewith the vertical scanning signal, by which the keystone eiect iseliminated. Both scanning signals are then arnplied by power ampliiiers'l and 8 and used to energize appropriate deflecting units 9 and lil,such as those commonly associated with cathode ray tubes.

From the illustrated embodiments and the above description, it will beseen that the invention provides novel means for the accomplishment ofthe objects and functions herein set forth. It will be understood, ofcourse, that the present disclosure is merely illustrative and imposesno limitation upon the invention, the scope of which is dened in theappended claims;

I claim:

1. In an electrical system, a plurality oi groups of space dischargedevices, each of said discharge devices having an input circuit and anoutput circuit, each of said discharge devices having an electrodecommon to its input and output circuits, each of said discharge devicesof one of said groups having a mutual conductance dependent upon thevoltage in its input circuit, another space discharge device having aninput circuit and an output circuit, an impedance having a voltagethereacross in the input circuits and output circuits of said one groupof discharge devices and said other discharge device, for establishingsaid voltage in each of said lastnamed circuits, means for introducingan input acer/,81o

signal in the input circuit of at least one of the space dischargedevices of said one group, means for introducing an input signal in theinput circuit of said other space discharge device, means including animpedance in the output circuit of one of the discharge devices of saidone group for deriving a signal therefrom, means including an impedancein the output circuit of another discharge device of said one group forderiving a second signal therefrom, means for connecting the outputcircuits of the discharge devices of another group in series relation,means including an impedance effectively -connected for signal currentsto each of the common electrodes of said last-named discharge devicesfor forming an output signal, means for applying said rst derived signalto the input circuit of one of said last-named ischarge devices, andmeans for combining said second derived signal and said output signaland for applying said combined signal to the input circuit of another ofsaid lastnarned discharge devices.

2. In an electrical system, a plurality of groups of space dischargedevices, each of said discharge devices having an input circuit and anoutput circuit, each of said discharge devices having an electrodecommon to its input and output circuits, each of said discharge devicesof one of said groups having a mutual conductance dependent upon thevoltage in its input circuit, another space discharge device having aninput circuit and output circuit, an impedance having a voltagethereacross in the input circuits and output circuits of said one groupof discharge devices and in the output circuit oi said other dischargedevice, for establishing said voltage in each of said last-namedcircuits, means for introducing an input signal in the input circuit ofat least one of the space discharge devices of said one group, means forintroducing an input signal in the input cir-cuit of said other spacedischarge device, means including an impedance in the output circuit ofone of the discharge devices of said one group for deriving a signaltherefrom, means including an impedance in the output circuit of anotherdischarge device of said one group for deriving a second signaltherefrom, means for connecting the output circuits of the dischargedevices of another group in series relation, means including animpedance effectively connected for signal currents to each of thecommon electrodes of said last-named discharge devices for forming anoutput signal, means for applying said iirst derived signal to the inputcircuit of one of said last-named discharge devices, and means forcombining said second derived signal and said output signal and forapplying said combined signal to the input circuit of another oi saidlast-named discharge devices.

3. In an electrical system, a plurality of groups of space dischargedevices, the space discharge devices in each group having similarelectrical characteristics, each of said discharge devices having aninput circuit and an output circuit, each of said discharge deviceshaving an electrode common to its input and output circuits, each ofsaid discharge devices of one of said groups having a mutual conductancedependent upon the voltage in its input circuit, another space dischargedevice having an input circuit and an output circuit, an impedancehaving a voltage thereacross in the input circuits and output circuitsof said one group of discharge devices and said other discharge device,for establishing said voltage in each of said last-named circuits, meansfor introducing an input signal in the input circuit of at least one ofthe space discharge devices of said one group, means for introducing aninput signal in the input circuit of said other space discharge device,means including an impedance in the output circuit of one of thedischarge devices oi said one group for deriving a signal therefrom,means including an impedance in the output circuit of another dischargedevice of said one group for deriving a .second signal therefrom, meansfor connecting the output circuits of the discharge devices ci anothergroup in series relation, means including an impedance effectivelyconnected for signal curents to each of the common electrodes of saidlast-named discharge devices for forming an output signal, means forapplying said first derived signal to the input circuit of one of saidlast-named discharge devices, and means for combining said secondderived signal and said output signal and for applying said combinedsignal to the input circuit of another of said last-named dischargedevices.

i. In an electrical system, a plurality of groups of space dischargedevices, the space discharge devices in each grouphaving similarelectrical characteristics, each of said discharge devices having aninput circuit and an output circuit, each of said discharge deviceshaving an electrode common to its input and output circuits, each ofsaid discharge devices of one of said groups having a mutual conductancedependent upon the voltage in its input circuit, another space dischargedevice having an input circuit and an output circuit, an impedancehaving a voltage thereacross in the input circuits and output circuitsof said one group of discharge devices and said other discharge device,for establishing said voltage in each of said last-named circuits, meansfor introducing an input signal cluding an impedance in the outputcircuit of one or the discharge devices of said one group for deriving asignal therefrom, means including an impedance in the output circuit ofanother discharge device of said one group for deriving a second signaltherefrom, said last-named impedances being substantially equivalent oneto the other, means for connecting the output circuits of the dischargedevices of another group in series relation, means including animpedance effectively connected for signal currents to each of thecommon electrodes of said last-named discharge devices for forming anoutput signal, means for applying said iirst derived signal to the inputcircuit of one of said last-named discharge devices, and means forcombining said second derived signal and said output signal and forapplying said combined signal to the input circuit of another of saidlast-named discharge devices.

5. in an electrical system, a plurality of groups of space dischargedevices, each of said discharge devices having an input circuit and anoutput circuit, each of said discharge devices having an electrodecommon to its input and output circuits, each of said discharge devicesof one of said groups having a mutual conductance dependent upon thevoltage in its input circuit, another space discharge device having aninput circuit and an output circuit, an impedance having a voltagethereacross in the input circuits and output circuits of said one groupof discharge devices and said other discharge device, ior establishingsaid voltage in each of said last-named circuits, means for introducingan input signal in the input circuit of one of the space dischargedevices of said one group and for introducing an input signal ofopposite phase relation in the input circuit of another of the spacedischarge devices of said one group, means for introducing an inputsignal in the input circuit of said other space discharge device, meansincluding an impedance in the output circuit of one of the dischargedevices of said one group for deriving a signal therefrom, meansincluding an impedance in the output circuit of another discharge deviceof said one group for deriving a second signal therefrom, means forconnecting the output circuits of the discharge devices of another groupin series relation, means including an impedance eiiectively connectedfor signal currents to each of the common electrodes of said last-nameddischarge devices for forming an output signal, means for applying saidrst derived signal to the input circuit of one oi said last-nameddischarge devices, and means for combining said second derived signaland said output signal and for applying said combined signal to theinput circuit ci another of said last-named discharge devices.

6. lin an electrical system, a plurality of groups of space dischargedevices, the space discharge devices in each group having similarelectrical characteristics, each of said discharge devices having aninput circuit and an output circuit, each of said discharge deviceshaving an electrode common to its input and output circuits, each ofsaid discharge devices of one of said groups having a mutual conductancedependent upon the voltage in its input circuit, another space dischargedevice having an input circuit and an output circuit, an impedancehaving a voltage thereacross in the input circuits and output circuitsof said one group of discharge devices and said other discharge device,for establishing said voltage in each of said last-named circuits, meansfor introducing an input signal in the input circuit of one of the spacedischarge devices of said one group and for introducing an input signalof opposite phase relation in the input circuit or another of the spacedischarge devices of said one group, means for introducing an inputsignal in the input circuit of said other space discharge device, meansincluding an impedance in the output circuit of one of the dischargedevices of said one group for deriving a signal therefrom, meansincluding an impedance in the output circuit of another discharge deviceoi said one group for deriving a second signal therefrom, saidlast-named impedances being substantially equivalent one to the other,means for connecting the output circuits of the discharge devices ofanother group in series relation, means including an impedance eectivelyconnected for signal currents to each of the common electrodes of saidlast-named discharge devices for forming an output signal, means forapplying said rst derived signal to the input circuit of one of saidlast-named discharge devices, and means for combining said secondderived signal and said output signal and for applying said combinedsignal to the input circuit of another of said last-named dischargedevices.

'7. In an electrical system, a plurality oi groups of space dischargedevices, the space discharge devices in each group having similarelectrical characteristics, each of said discharge devices having aninput circuit and an output circuit, each of said discharge deviceshaving an electrode common to its input and output circuits, each ofsaid discharge devices of one of said groups having a mutual conductancedependent upon the voltage in its input circuit, another space dischargedevice having an input circuit and an output circuit, an impedancehaving a voltage thereacross in the input circuits and output circuitsof said one group of discharge `devices and said other discharge device,for establishing said voltage in each of said last-named circuits, meansfor introducing an input signal in the input circuit of at least one ofthe space discharge devices oi said one group, means for introducing aninput signal in the input circuit of said other space discharge device,means including an impedance in the output circuit of one of thedischarge devices of said one group for 'deriving a signal therefrom,means including an impedance in the output circuit of another dischargedevice of said one group for deriving a second signal therefrom, saidlast-named impedances being substantially equivalent one to the other,means for connecting the output circuits of the discharge devices ofanother group in series relation, means including an impedanceeffectively connected for signal currents to each of the commonelectrodes of saidV last-named discharge devices for forming an outputsignal, means including a transfer circuit having a certain frequencyresponse for applying said rst derived signal to the input circuit ofone of said last-named discharge devices, and means including anothertransfer circuit having a substantially similar frequency response forcombining said second derived signal and said output signal and forapplying said combined signal to the input circuit of another of saidlast-named discharge devices.

8. In an ampliiier circuit, a balanced or double-sided circuit, anunbalanced or singlesided load impedance for said amplifier, a pair ofvacuum tubes, input circuits for said tubes coupled to said balancedcircuit, a direct currentV supply source for said tubes, means forconnecting the direct current plate circuits of said tubes in serieswith respect to one another and said direct current source, and meansfor connecting the signal output circuits of said tubes in parallel withrespect to said load impedance, said load impedance being included inthe direct current plate circuits of both of said tubes and in the inputcircuit of one of said tubes.

9.'In an amplifier circuit, a pair of space discharge devices eachhaving a cathode, anode, and control grid, signal input circuits forsaid devices, a source'of direct current, means connecting said sourceand the anode-cathode paths of said devices in a series loop circuitwith respect to the direct current supplied by said source, and analternating-'current output impedance for said devices, said impedancebeing connected by a degenerative cathode load for one of said devicesand as an anode load for the other of said devices.

10. In an amplifier circuit, a pair of space discharge devices eachhaving a cathode, anode, and control grid, a source of direct current,means connecting said source and the anodecathode paths of said devicesin a series loop circuit with respect to the direct current supplied bysaid source, an alternating-current output impedance for said devices,said impedance being connected as a cathode load for one of said devicesand as an anode load for the other of said devices, means for applyinganinput signal to the grid-cathode circuit of one of said devices, andmeans for combining another input signal with the output signal acrosssaid outp-ut impedance and for applying said combined signal to thegrid-cathode circuit of the other of said devices.

l1. In an amplifier circuit, a pair of space discharge devices eachhaving a cathode, anode, and control grid, a balanced input circuitconnected to said control grids forr driving said grids in oppositephase relation, a source of direct current, means connecting said sourceand the anode-cathode paths of said devices in a series loop circuitwith respect to the direct current supplied by said source, analternating-current output impedance for said devices, and connectionsfor shunting the anode-cathode paths of said devices across said outputimpedance in mutually reversed relation, said output impedance beingincluded in the direct current anodecathode circuits of both of saiddevices and in the grid-cathode circuit of one of said devices.

l2. In a signal transfer circuit, a pair of space discharge devices eachhaving a grid-cathode input circuit and an anode-cathode output circuit,a source of direct current, means connecting said source and theanode-cathode paths of said devices ina series loop circuit, and acommon load impedance for said serially connected devices, said loadimpedance being disposed in both the anode-cathode and grid-cathodecircuits of one of said devices, but only in the anodecathode circuit ofthe other of said devices.

13. In a signal combining circuit, a pair of space discharge deviceseach having a gridcathode input circuit and an anode-cathode outputhcircuit, a source of direct current, means connecting said source andthe anode-cathode paths of said devices in a series loop circuit, acommon load impedance for sai-d devices for forming an output signalthereacross, means for applyingV an input signal to one of said inputcircuits,V and means for combining an input sig-` nal with said outputsignal and for applying said combined signal to the other of said inputcircuits.

DAVID B. SMITH.

